1. Field of the Invention
The present invention relates to a lens system assembling method for assembling a lens system by fixing the positions of two mutually opposing lenses such that the spacing between the two lenses is set at a predetermined distance. The invention also relates to a spacing jig used for the lens system assembling method.
2. Description of the Related Art
The objective lens mounted on an endoscope, a microscope, or the like includes a plurality of lenses disposed in a lens barrel. A method for setting the spacing (also referred to as “air-spacing”) between the lenses using a cylindrical spacer ring is known as described, for example, in Japanese Unexamined Patent Publication No. 2003-262774 and U.S. Patent Application Publication No. 20040158129. It is also known that lens setting errors may be within the range from ±0.01 to 0.02 mm by disposing the spacer ring between two lenses and abutting the two lenses to an edge portion of the end faces of the spacer ring. For example, if the spacer ring is disposed between two convex lenses, the lens faces contact the inner edge portion of the end faces of the cylindrical spacer ring, and if the spacer ring is disposed between two concave lenses, the lens faces contact the outer edge portion of the end faces of the spacer ring. Therefore, spacing errors between lenses are mainly dependent on the dimensional accuracy of the spacer ring in the cylinder axis directions and the dimensional accuracy of the inner or outer diameter of the spacer ring.
In the mean time, there is a demand that the spacing between two mutually opposing lenses in an objective lens, which includes a plurality of lenses, be set more accurately, in order to improve the performance of the objective lens. On the other hand, spacer rings are parts to be incorporated and used in objective lenses, and are expected to be produced at a low cost, which inevitably sets a limit on the dimensional accuracy of the spacer rings. In particular, improvement in the dimensional accuracy of the inner diameter is difficult. There is also a problem that the setting error of the spacing between lenses increases as the ratio of the radius D/2 (D is the diameter) of a lens in the direction orthogonal to the optical direction of the lens to the radius of curvature R of the lens face, i.e., the value of D/(2×R) (DR value) becomes closer to 1 (approaches closer to a hemisphere), which results from a dimensional error of the inner or outer diameter of the spacer ring.
The problems described above are not limited to objective lenses, and common to general lens systems in which the spacing between two mutually opposing lenses is set using the spacer ring.